Internally threaded, plastic cap closures have found widespread application for use in connection with hot-fill plastic containers by virtue of their low manufacturing costs and sealing performance. In a conventional hot-fill process, a hot beverage product is introduced into the plastic container, typically filling most of the container. The fluid is heated during a pasteurization or sterilization process to remove bacteria or other contamination. The plastic container is hermetically sealed with a cap while the product is still hot. Since the beverage product is typically not filled to the top of the container, a headspace of air is provided between the liquid enclosed within the plastic container and an inner surface of the cap. The temperature of the liquid varies from a high of about 185 degrees Fahrenheit, the typical hot-fill temperature, to about 40 degrees Fahrenheit, the typical refrigeration temperature. A change in temperature, from hot to cold, decreases the internal pressure of the sealed container and creates a vacuum within the container primarily as a result of the thermal contraction of the liquid in the container. This decrease in pressure can distort and/or deform the geometry of the container if the container cannot structurally support the pressure difference between the external ambient pressure and the lower internal pressure of the container. Deformation of the container generally pushes the fluid upwardly and decreases the headspace volume. For example, for a typical 16-ounce container, thermal contraction equates to roughly 3% of the total liquid volume, or 0.9 cubic inches when the stored contents are cooled from about 185° to about 40° F.
Current containers are engineered to collapse at specific locations or are reinforced with vacuum panels and/or flexible bases to compensate for the vacuum. Vacuum-reactive mechanisms are very efficient to maintain a balanced pressure and keep the remaining structural geometry of the container from collapsing. Vacuum panels, however, are difficult to mold. Further, labeling of the container is difficult because containers employing raised and/or recessed vacuum panels possess reduced surface area. The reduction of surface area also restricts the ornamental design of the label, restricts the placement of the label, and often leads to unattractive wrinkling of the label.
Embodiments of the present invention described herein are directed to an apparatus and method for accommodating the pressure decrease associated with hot filling and subsequently cooling a liquid stored in a plastic container. By addressing the vacuum created within the container, vacuum panels may be eliminated or reduced.